FR3010083A1 - PROCESS FOR FILTERING A SOLUTION OF PLASTIC MATERIAL IN A SOLVENT - Google Patents

Abstract

Process for filtering a solution of plastic material in a solvent and containing insoluble particles, according to which: - this solution is passed through a filter element having a filtering surface - passes at least one scraper near the filtering surface of so as to accumulate insoluble particles in the vicinity of the scraper and to substantially sweep the entire filtering surface with these particles.

Description

The present invention relates to a process for filtering a solution of plastic material in a solvent. Plastics are widely used in various forms, mainly in the solid state, for the manufacture of various items, flexible or rigid, such as for example tarpaulins, coated fabrics and other elements for the interior trim of vehicles , pipes, blisters (pharmaceutical packaging), window frames or electrical cables with polymeric insulation. However, it often happens that at a given moment of their existence, they are in solution in a solvent from which it is then necessary to extract them. Thus, we are confronted with polymer solutions at the end of certain polymerization processes (called "in solution"), during certain recycling processes, during the cleaning of certain object-making or paint-based installations. of polymers ...

Thus, various processes for recycling plastics based on dissolution using organic solvents have been proposed. The patent application EP 945481 in the name of the applicant proposes for example a recycling process of an article based on at least one vinyl chloride polymer, according to which: (a) the article is shredded into fragments of an average dimension of 1 cm to 50 cm in case it exceeds these dimensions; (b) the dry article fragments are contacted with a solvent capable of dissolving the substantially anhydrous polymer, forming with water azeotrope (MEK preferably); (c) the precipitation of the polymer dissolved in the solvent is caused by the injection of water vapor into the solution thus obtained, which further causes the entrainment of the solvent-water azeotrope and thus leaves a mixture consisting essentially of water and solid particles of polymer; (d) the polymer particles are collected.

Such a method generally also comprises at least one of the following steps: a: the fragments of articles are fed from a storage silo to a dissolver - 2 - b1: the polymer solution is fed from the dissolver to a primary filter separating the large insolubles (typically reinforcements or accessories having at least a dimension> 150 μm) of the polymer solution b2: the insolubles are preferably treated to remove traces of polymer and residual solvent, which can be done by stripping with steam b3: the polymer solution is sent to a precipitator by prior passage in a centrifugal decanter to remove insoluble smaller size cl: the polymer particles from the precipitator are separated from the water and brought to a dryer using a pump. Depending on the type of insoluble content in the article to be recycled, it may be advantageous to operate the steps b, bl, b2 and b3 in the same reactor, which is then a dissolver-filter. EP 2276801 in the name of the applicant describes a method using such an apparatus.

Step b1 described above is generally carried out using a pump. As regards the filter, it is advantageously a Nutsche type filter. This type of filter generally comprises a tank, a filter plate, arms for stirring the suspension and a mechanism for discharging the "cake" (filtrate or insoluble).

In this process, it is sometimes advantageous to introduce also or alternatively to the aforementioned centrifugal decanter, where appropriate, a secondary filter positioned just before the precipitator. This filter then advantageously contains a rotary filter element. With regard to the filter elements mentioned above, there are on the market plates or wedge-wire type cylindrical elements which give excellent results with plastic solutions comprising insolubles. This type of filter element, well known to those skilled in the art, comprises profiles, generally having a triangular section, which are welded to supports defining between them slots having a triangular vertical section whose "tip" (which is actually flattened) is flush with the filtering surface. Companies manufacturing this type of filter element (wedge-wire) are for example Trislot and Euroslots kdss. These elements have a lower tendency to clog compared to conventional mesh filters by the geometry (conical) of their slots. This same advantage is present with conical hole plates also having a triangular vertical section whose "tip" is flush with the filtering surface. - 3 - However, insoluble impurities present in certain plastic waste have a geometry such (often filamentous) that they manage nevertheless to plug the conical slots / openings: cf the waste of electrical cables for example, which often include ( pieces of) copper filaments; see also wallpaper waste, carpet etc. which often contain (pieces of) fibers. The application JP 2009-149766 describes a method for avoiding the clogging of filter elements used for solutions of such waste and which consists of scraping the filter elements using a rotary blade.

The present invention is based on the surprising finding that it is not necessary for the blade to directly scrape the filter elements and, on the contrary, to provide a space between the blade and the filter element which is sufficiently small that to create an accumulation of insoluble impurities, it is these that scrape the filter element and with increased efficiency. Therefore, the present invention relates to a process for filtering a solution of plastic material in a solvent and which contains insoluble particles, wherein: - this solution is passed through a filter element having a filter surface - one passes at least one wiper near the filtering surface so as to accumulate insoluble particles in the vicinity of the wiper and to substantially sweep the entire filtering surface with these particles. The plastics material of the present invention may be of any kind. It may be an apolar polymer, such as a polymer of ethylene (PE) or propylene (PP); styrene polymers such as PS (polystyrene) ABS (acrylonitrile-butadiene-styrene terpolymer). It may also be a polar polymer such as a polymer of vinyl chloride (PVC) or vinylidene chloride (PVDC), or a copolymer of ethylene and polyvinyl alcohol (EVOH). It can also be a mixture of such plastics of which at least one is in solution. Good results have been obtained with PVC. By PVC is meant any homo- or copolymer containing at least 50% by weight of vinyl chloride. Before being dissolved, the plastic material may be in any form. It may for example be waste polymerization, compounding or implementation, possibly in the liquid or pasty state, possibly even in solution in a solvent. It may also be solid articles (in the form of flexible or rigid pipes, containers, sheets for covering floors, tarpaulins, window frames, electrical cable insulation sleeves, etc.). ) comprising one or more usual additives such as, for example, plasticizers, stabilizers, antioxidants, flame retardants, pigments, fillers, etc., including reinforcing fibers. These fibers can be of any nature, natural or synthetic; in particular, glass, cellulose or plastic fibers, such as polyester fibers and, in particular, polyethylene terephthalate (PET) fibers can be used.

The insolubles generally contained in the solution (and generally present in the aforementioned articles) may be (fragments of) reinforcements (fibers, various fillers), "accessories" such as metal eyelets, labels, metal conductors, etc. ; plastic materials of another nature ... For example, waste electrical cables generally contain PVC and copper; roof membrane waste generally comprises PVC and PET fibers; Pharmaceutical blister waste generally comprises several different plastics (typically PVC and PE), which are not soluble in the same solvents; wallpaper waste often contains PVC and paper etc. by definition, these insolubles are present as solid particles in the solution. Note that there may be several types of insolubles as described above in the same solution. The present invention gives good results with a plastic solution obtained from waste electric cables. In a process where the choice of the solvent capable of dissolving the plastic material is free (recycling process for example), it is preferably selected from liquids having a solubility parameter (a definition and experimental values of which appear in " Properties of Polymers ", DW Van Krevelen, 1990 Edition, pp.200-202, as well as" Polymer Handbook ", J. Brandrup and EH Immergut, Editors, Second Edition, pp. IV-337 to IV-359) the solubility parameter of the plastic material to be dissolved and / or having strong interactions with it (eg hydrogen bonds). The term "neighbor" is generally equivalent to "not deviating more than 6 units". It is generally an organic solvent, preferably polar such as MEK (methyl ethyl ketone), which gives good results with many polymers and in particular with halogenated polymers such as PVC. It is understood that by solvent are meant both simple substances and mixtures of substances. In particular, in the case of a continuous and / or closed loop recycling process, the solvent may be a recycled liquid stream and may contain a certain amount of non-solvent. According to the invention, the plastic solution is passed through a filter element. When it is highly charged insoluble (which is typically the case of the aforementioned primary filter), this is advantageously using a vortex pump. The filter element may be in the form of a plate, a cylinder or drum (filter cartridge)... It is advantageously provided with openings (holes, slots) having a triangular vertical section and very particularly preferably, whose "tip" is flush with the filtering surface. In a very particularly preferred manner, it is of the wedge-wire type. According to the invention, at least one scraper is passed close to the filtering surface.

By "scraper" is meant an object of generally elongate shape (such as an arm, a blade, a metal sheet, etc.) which preferably extends radially from the center of the filter element towards its periphery and which is subjected to to a rotational movement about an axis passing through said center. This object may possibly be a sheet attached to at least one of the blades of an agitator (when the filter is provided with this, which is frequently the case). If necessary, several scrapers may be used whose geometry and location are preferred such that substantially all of the filtering surface is cleaned, i.e. at least 90% of the surface and if possible: its entirety. By "near" is meant that the scraper is not in direct contact with the filter element but that there is a space / slot, preferably of constant height, between the scraper and the filtering surface, this height being preferably between 0.5 and 30 mm, preferably between 1 and 15 mm. To avoid friction and associated wear problems, the scraper is preferably (at least in its surface layer in contact with the insoluble) based on PTFE (polytetrafluoroethylene sometimes called Teflon) and preferably graphite-filled PTFE . In one variant, the process according to the invention also uses a doctor blade (comb, blade, etc.) as described in the above-mentioned application W, the contents of which are incorporated in the present application, and which is in direct contact with the present invention. the filtering surface. This variant makes it possible to cut the particles (filaments generally) which would still be stuck in the orifices of the filtering element and, in doing so, to facilitate their subsequent dislodgement. The process according to the invention is preferably incorporated into a plastic recycling process as described above. To this end, the present invention also relates to a process for recycling (fragments) of plastic articles by dissolution / precipitation, according to which before precipitation of the plastic material, the solution obtained by dissolving the (fragments of) articles is filtered as previously described. The characteristics of the articles, the plastic material, the solvent, the filter element, the scraper, etc. are as previously described. In a first variant of the invention, the filter element is that of a primary filter which filters the polymer solution directly from the dissolution of the article fragments in the solvent. In a first sub-variant, this filter is located directly downstream of a dissolver where the article fragments are dissolved in the solvent. This filter is preferably of Nutsche type. It is preferably equipped with a wedge-wire type filter element. The solution is preferably brought from the dissolver by a vortex pump. In a second sub-variant, the filter element is located inside the dissolver itself. It is then advantageously provided with a manhole removable from the outside and which maintains the filter element, which is preferably a disk-shaped plate. The scraper or scrapers that sweep the plate are then advantageously attached to the lower blades of the stirrer which is generally present in a dissolver. The advantage of such a geometry is to allow the disassembly of the plate to clean it. Indeed, since a dissolver-filter is generally used to also, in situ, stripping the insoluble, the orifices of the plate tend to clog. In a second variant of the invention, the filter is a secondary filter located between a primary filter as described above and a precipitator. This filter preferably comprises a rotary filter cartridge equipped with wedgewires (or alternatively: conical holes). It preferably comprises two scrapers, preferably made of graphite-filled PTFE, and a scraper directly in contact with the filtering surface (as described above), preferably of a material which is less hard than the constituent material of the cartridge but harder than the constituent material of insolubles. This is for example a Cu-Be alloy harder than copper cable waste but less hard than steel (usually 316L stainless) which is usually made the cartridge. This combination allows both to release the slots / holes of the cartridge (by the scrapers) and cut the insoluble (copper wire for example) that would still be stuck.

Claims (9)

CLAIMS 1 - Process for filtering a solution of plastic material in a solvent and which contains insoluble particles, according to which: - this solution is passed through a filter element having a filtering surface - passes at least one scraper close to the surface filtering so as to accumulate insoluble particles in the vicinity of the scraper and to substantially sweep the entire filtering surface with these particles. 2 - Process according to claim 1, wherein the plastic material is a polymer of vinyl chloride (PVC) and the solvent is MEK (methyl ethyl ketone). 3 - Process according to any one of the preceding claims, wherein the filter element is provided with openings having a triangular vertical section whose tip is flush with the filter surface. 4 - Process according to any one of the preceding claims, characterized in that the scraper is a plate attached to at least one of the blades of a stirrer. 5 - Process according to any one of the preceding claims, characterized in that the scraper is PTFE loaded with graphite. 6 - Process according to any one of the preceding claims, characterized in that one also uses a doctor blade in direct contact with the filtering surface. 7 - Method for recycling plastic articles by dissolution / precipitation, according to which before precipitation of the plastic material, the solution obtained by dissolving article (s) or fragments of article (s) is filtered by a method according to any one of the preceding claims. 8 - Process according to the preceding claim, wherein the filter element is that of a primary filter which filters the polymer solution directly- 9 - result of the dissolution of the article (s) or fragments of article (s) in a dissolver. 9 - Process according to claim 8, wherein the filter element is located inside the dissolver which is preferably provided with a manhole removable from the outside and which maintains the filter element. - The method of claim 7, wherein the filter element is that of a secondary filter located between a primary filter filtering the polymer solution directly from the dissolution, and a precipitator.